1) Field of the Invention
The present invention relates to surgical procedures for stabilizing the spine, restoring disc height and reestablishing anatomic curves of the spine and, more particularly, to a spinal implant device for use in such procedures.
2) Description of the Prior Art
The human spinal column includes over twenty bones or vertebrae. Spinal disc cartilage is positioned between the adjacent vertebrae. Through the wear and tear of everyday living, damage can occur between the spinal disc cartilages positioned between the adjacent vertebrae or the vertebra itself. This degeneration can cause excessive back pain.
A number of procedures and devices has been developed to correct this problem, such as interbody fusion devices. One of the most popular interbody fusion devices has taken on the form of a cylindrical implant, such as described in U.S. Pat. Nos. 4,501,269; 4,743,256; 4,834,757; 4,878,915; 4,961,740; 5,015,247; 5,055,104; and 5,192,327. These cylindrical implants can either be threaded or pounded into the disc space between the adjacent vertebrae.
In each of the above-identified patents, the cross section of the implant is constant throughout its length and is typically in the form of a right circular cylinder. An advantage of the circular design is that current surgical drills can easily drill a substantially circular profile into a bone. The bone is more difficult to prepare for other profiles.
However, one problem encountered with these prior art devices is that they do not maintain or restore the normal anatomy of the fused spine segment. In other words, once the disc is removed, the normal lordotic or kyphotic curvature of the spine is eliminated.
Several attempts have been made to provide implants that attempt to restore the curvature of the spine. U.S. Pat. Nos. 5,669,909 and 5,683,463 provide one-piece, frusto-conical shaped implants. Although in theory these implants may restore the curvature of the spine, they are difficult to install since special reamers may be needed to provide a tapered hole or a cylindrical hole must be modified by the implant which could put undue stress on the adjacent vertebrae.
U.S. Pat. No. 5,653,763 describes a multi-piece rectangular-shaped dynamic implant that includes rectangular-shaped legs hinged at one end. An arrangement for separating the legs is provided that includes a shaft and a nut received within an interior space defined by the legs. This device requires special surgical tools to prepare a rectangular space between the adjacent vertebrae. Further, the hinged body may affect the integrity of the implant over time. Also, this device has no areas to receive bone mass to facilitate healing.
U.S. Pat. No. 5,554,191 discloses an intersomatic vertebrae cage inserted from the posterior approach between two vertebra. This arrangement includes an integral body having two legs with a multi-piece adjustment mechanism to adjust the spacing between the legs. The cage is rectangular in cross section and has no areas defined to receive bone mass to facilitate healing.
Therefore, it is an object of the present invention to provide a spinal implant device that is relatively easy to install either posteriorly or anteriorly, maintains the appropriate curvature of the spine and facilitates healing.